1. Field of the Invention
The present invention relates to a preparation process and a purification process of a cyclic ester typified by glycolide, lactide and tetramethyl glycolide. More specifically, it relates to a process for easily preparing a high-purity cyclic ester having extremely low contents of impurities such as an acid and water in a high yield by the use of an orthoester, and a process for purifying the cyclic ester.
2. Description of the Related Art
Cyclic esters typified by glycolide and lactide are cyclic dimers produced by the dehydration of two molecules of an .alpha.-hydroxycarboxyliclic acid or its ester, or by the removal of an alcohol therefrom, and they fall into the category of intramolecular cyclic esters. Thus, poly(.alpha.-hydroxycarboxyliclic acid esters) typified by poly(glycolic acid) and poly(lactic acid) obtained by the ring opening polymerization of these intramolecular cyclic esters have excellent biological degradation properties, so that they have been utilized as biologically degradable materials. In recent years, the problem of plastic wastes occurs owing to the shortage of garbage reclaiming lands and the incineration of the wastes, and so the poly(.alpha.-hydroxycarboxylic acid esters) are expected as biologically degradable plastics which can be degraded by hydrolysis, enzymes and microorganisms. In consequence, the demand and supply of these polyesters are gradually increasing.
In order to achieve a high molecular weight required for the biologically degradative materials and the biologically degradative plastics, it is necessary that the content of impurities such as an acid and water in a cyclic ester which is a starting material should be controlled to the lowest possible level, because such impurities disturb the attainment of the high molecular weight necessary to obtain sufficient physical properties. In the case of lactide, for example, it has been expected that the acid content is preferably 10 meg/kg or less, more preferably 5 meg/kg or less, and the water content is preferably 200 ppm or less, more preferably 100 ppm or less.
As a process for preparing a cyclic ester such as lactide of another .alpha.-hydroxycarboxyliclic acid or .alpha.-hydroxycarboxyliclic acid ester, there is usually a process for preparing the cyclic ester which comprises polymerizing a corresponding .alpha.-hydroxycarboxyliclic acid or .alpha.-hydroxycarboxyliclic acid ester to form a poly(hydroxycarboxylic acid) or a poly(hydroxycarboxylic acid ester) which is an oligomer, heating this oligomer up to about 200.degree. C. in vacuum in the presence of a catalyst known in this technical field which is typified by stannous octanoate to decompose the same, whereby a cyclic dimer is obtained, and then distilling off the thus obtained cyclic ester (e.g., U.S. Pat. Nos. 1,095,205, 2,668,162, 4,797,468 and 5,053,522, Japanese Patent Application Laid-open Nos. 101378/1988 and 268179/1990, and Japanese PCT Patent Application Laid-open Nos. 503490/1995 and 505150/1995).
However, in the cyclic ester obtained by such a process, volatile impurities such as water and an acidic hydroxyl compound are included (refer to Comparative Example 3). Examples of these impurities usually include water, an .alpha.-hydroxycarboxyliclic acid monomer and oligomers of the .alpha.-hydroxycarboxyliclic acid. As described above, water and the carboxylic acids function as polymerization inhibitors during the polymerization of the cyclic ester, and they also deteriorate the storage stability of the cyclic ester. Therefore, the presence of these impurities is not preferable. Furthermore, Japanese PCT Patent Application Laid-open No. 503490/1995 and 505150/1995 have disclosed methods which comprise removing the hydroxyl impurities by redistillation to obtain the high-purity cyclic ester, but these methods have many disadvantageous points with respect of industrialization. For example, the yield of the cyclic ester which can be used as a polymer material deteriorates; the cyclic ester can scarcely possess such a quality as to be usable as the polymer material, even when the redistillation is done; operation is intricate; and facilities for the redistillation are additionally required (refer to Comparative Example 4).
In Japanese PCT Patent Application Laid-open No. 504916/1995, a process for preparing the cyclic ester from hydroxy acid and its derivative has been described, in which existing water is removed by the use of water getters such as acetic anhydride, an acetal and a carbodiimide in a step. However, these compounds have not been used in examples of the above-mentioned publication, and so the effect of the compounds is indefinite. Furthermore, in view of a fact that acetic acid is produced from acetic anhydride, the employment of acetic anhydride results in a reverse effect, instead of the reduction of an acid value. According to experiments of the acetal by the present inventors, the effect of the acid value reduction has not been observed (refer to Comparative Example 6). The carbodiimide has both the effects of the water removal and the acid value reduction, but since it contains a nitrogen atom, the carbodiimide causes the coloring of the cyclic ester, and it is also very expensive. For these reasons, it is difficult to industrially use the carbodiimide. Also in Japanese PCT Patent Application Laid-open No. 504762/1994, some water getters are referred to, but it is apparent that they also have the above-mentioned drawbacks.
In Japanese Patent Application Laid-open Nos. 287056/1993 and 287278/1994, a manufacturing process of an aliphatic polyester has been described which comprises polycondensing an .alpha.-hydroxycarboxyliclic acid in the presence of a primary alcohol, a carbodiimide or a polyhydric alcohol having 3 or more hydroxyl groups (a carboxyl group concentration adjustor) to form a precursor polymer having a carboxyl group concentration of 200 meg/kg or less, heating the precursor polymer to depolymerize it, and then subjecting the resulting cyclic ester to ring opening polymerization. Furthermore, Japanese Patent Application Laid-open No. 309862/1995 has described a process for preparing lactide which comprises dehydrocondensing lactic acid, to which a polyhydric alcohol having 3 or more hydroxyl groups has been added, in the presence of a metallic compound having the electronegativity of a metallic ion in the range of 10 to 15 to synthesize a precursor polymer having a carboxyl group concentration of 200 meg/kg or less, and heating this precursor polymer to depolymerize it. However, the cyclic ester (lactide) obtained by these methods also has some drawbacks. For example, the cyclic ester (lactide) does not possess such an acid value that it withstands a use as a polymer; the purity of the cyclic ester (lactide) is low and so a purification step is necessary for the use as the polymer; and the carboxyl group concentration adjustor and a substance formed by the modification of the adjustor with a reaction always remain in a reaction solution, which fact might cause the coloring of the obtained cyclic ester (lactide).
As techniques of purifying the cyclic ester, there are known recrystallization methods in which a sufficiently dried organic solvent such as toluene, ethyl acetate or isopropanol, or a mixed solvent is used (e.g., Japanese Patent Application Laid-open No. 101378/1988, Japanese PCT Patent Application Laid-open No. 507076/1993, and Japanese Patent Application Laid-open Nos. 279435/1994, 1,182,59/1995 and 2,068,51/1995; and as methods similar to the recrystallization methods, Japanese Patent Application Laid-open No. 165430/1988, and Japanese Patent Publication Nos. 25912/1990 and 15712/1993). The cyclic ester, from which the impurities for disturbing the polymerization have been removed by the recrystallization method and which has sufficiently been purified, can be used as a monomer for the polymerization, after subjected to steps such as filtration and drying. However, such a recrystallizing operation leads to the deterioration of the yield of the cyclic ester, makes the procedure intricate, and increases costs due to facilities required for the recrystallizing operation. Hence, the recrystallization methods have many disadvantageous points which make industrialization difficult.
Japanese Patent Application Laid-open No. 256340/1994 has suggested a purification process of lactide which comprises (1) cooling a molten mixture of lactide and impurities to the solidifying point of lactide or a little lower temperature than the solidifying point of lactide, (2) partially crystallizing the molten mixture to form a solid phase having a lower impurity content than the molten mixture and a liquid phase having a higher impurity content than the molten mixture, and (3) separating the solid phase from the liquid phase to collect the solid phase. This process has succeeded in obtaining lactide having an acid value of about 2 meq/g which can be used as a polymer material. However, this process also has some drawbacks. For example, temperature control is difficult in melting a solid lactide mixture and in cooling the molten mixture to the solidifying point of lactide; productivity is poor for reasons of a low zone movement rate and the like; and a facility cost increases uneconomically. Hence, this process has many disadvantageous points which make it difficult to actually industrialize the process.
In Japanese Patent Application Laid-open No. 223248/1991, a process for preparing an unsaturated dicarboxylic acid imide compound has been disclosed which comprises dehydrating an unsaturated dicarboxylic acid amide acid compound in the presence of an acid anhydride as a dehydrating agent, a basic catalyst and a metallic salt catalyst to close its ring, and then decomposing a secondarily produced acid with an orthoester into an alcohol and an ester to remove the acid, thereby obtaining the unsaturated dicarboxylic acid imide compound. However, in this publication, the preparation method of the unsaturated dicarboxylic acid imide compound is only described, and any description regarding the process for preparing the cyclic ester is not referred to therein.